Generally, when a wellbore for oil or gas production is created, a number of casings are installed in the borehole. These casings serve to prevent collapse of the borehole wall and to prevent undesired outflow of drilling fluid into the formation or inflow of fluid from the formation into the borehole from strata other than the target production strata. The borehole is drilled in intervals whereby each casing is installed after drilling of a next interval, so that a next casing to be installed is to be lowered through a previously installed casing. In a conventional method of creating a wellbore the outer diameter of the next casing is limited by the inner diameter of the previously installed casing in order to allow lowering of the next casing through the previous casing. Thus, the casings are nested relative to each other, with casing diameters decreasing in downward direction. Cement annuli are provided between the outer surfaces of the casings and the borehole wall to seal the annuli between the casing and the borehole wall. As a consequence of the nested arrangement of the casings, a relatively large borehole diameter is required at the upper part of the wellbore. Such a large borehole diameter involves increased costs due to heavy casing handling equipment, large drill bits and increased volumes of drilling fluid. Moreover, increased drilling rig time is involved due to required cement pumping and cement hardening.
It is an object of the invention to provide a method of creating a wellbore in an underground formation, which method eliminates the need for a relatively large borehole diameter in the upper part of the wellbore and thereby overcomes the disadvantages of the conventional method.